Light emitting apparatuses using a combination of a semiconductor light emitting element and a phosphor have been attracting attention as next-generation light emitting apparatuses expected to realize low power consumption, compact size, high luminance, high color gamut, and high color rendition, and have been actively researched and developed. Primary light emitted from a light emitting element in a range from ultraviolet light having a longer wavelength to blue light, i.e., 380 to 480 nm, is usually used. Light converters using various phosphors suitable for this application have also been proposed.
At present, for white light emitting apparatuses of this type, a combination of a light emitting element emitting blue light (peak wavelength: around 460 nm) and a trivalent cerium-activated (Y, Gd)3(Al, Ga)5O12 phosphor or a divalent europium-activated 2(Sr, Ba)O.SiO2 phosphor, which is excited by the blue color and emits yellow light, is mainly used. However, color gamut (NTSC ratio) in such light emitting apparatuses is about 70%, and higher color gamut has recently been required even in compact LCDs.
Further, for the light emitting apparatuses of this type, an attempt has recently been made not only to improve conversion efficiency (brightness) but also to increase input energy to achieve higher brightness. When input energy is increased, it becomes necessary to efficiently dissipate heat from an entire light emitting apparatus including a light converter. Thus, development of the structure of the entire light emitting apparatus, materials therefor, and the like has been pursued. However, a temperature rise in the light emitting element and the light converter during operation is still inevitable.
However, the trivalent cerium-activated (Y, Gd)3(Al, Ga)5O12 phosphor particularly has a technical problem that it is impossible to set high input energy, because the luminance (brightness) at 100° C. decreases to about 85%, compared to the luminance of 100% at 25° C. Therefore, it is also urgently necessary to improve the temperature characteristic of phosphors to be used for the light emitting apparatuses of this type.
For these technical problems, it is known that the use of a divalent europium-activated oxynitride green light emitting phosphor which is a β-type SiAlON represented by EuaSibAlcOdNe results in a light emitting apparatus having good color gamut (NTSC ratio) and temperature characteristic.
However, the divalent europium-activated oxynitride green light emitting phosphor which is a β-type SiAlON is basically a columnar crystal, and, due to its simple composition, if an attempt is made to maintain its crystal structure, sintered bodies (aggregates) are likely to be generated. In an excessively large columnar crystal (needle crystal), crystal growth is insufficient, and a good characteristic (brightness) cannot be obtained. Further, since a sintered body (aggregate) is not a uniform single particle, a good characteristic (brightness) cannot be obtained despite its size, due to absorption of light at a particle boundary and the like. In particular, presence of many columnar crystals (needle crystals) and sintered bodies (aggregates) causes technical problems that sufficient brightness cannot be obtained in the light emitting apparatus, and that a reduction in brightness and large fluctuations in chromaticity are caused even during continuous lighting.
Therefore, it is urgently necessary to develop a divalent europium-activated oxynitride green light emitting phosphor which is a β-type SiAlON represented by EuaSibAlcOdNe having a controlled shape, and a light emitting apparatus using the same having a stable characteristic. Japanese Patent Laying-Open No. 2005-255895 (Patent Literature 1), for example, has descriptions regarding a β-type SiAlON that its crystal phase is a single crystal having an average particle diameter of not less than 50 nm and not more than 20 μm, and that the particle size of synthesized phosphor powder is adjusted to have an average particle diameter of not less than 50 nm and not more than 20 μm. Patent Literature 1 also describes that, if the average particle diameter is more than 20 μm, it is not preferable because dispersibility is deteriorated and color unevenness is caused when the phosphor powder is applied to an illumination device or an image display apparatus, and that, if the average particle diameter is less than 50 nm, the powder aggregates and causes deterioration in operability. However, Patent Literature 1 has no descriptions regarding dispersibility (i.e., the degree of aggregation (sintering)), regarding the relationship between dispersibility and an absorptance at 600 nm, and regarding the initial characteristic and the life characteristic of a light emitting apparatus.